1. Field of the Invention
A disclosed embodiment of the invention relates generally to the detection of errors in ultrasonic transit time measurements. More particularly, a disclosed embodiment of the invention relates to the identification of mistakes in peak selection and other errors for the ultrasonic meter, with another aspect of the invention relating to a method for correction of ultrasonic meter measurement errors.
2. Description of the Related Art
After a hydrocarbon such as natural gas has been removed from the ground, the gas stream is commonly transported from place to place via pipelines. As is appreciated by those of skill in the art, it is desirable to know with accuracy the amount of gas in the gas stream. Particular accuracy for gas flow measurements is demanded when gas (and any accompanying liquid) is changing hands, or “custody.” Even where custody transfer is not taking place, however, measurement accuracy is desirable.
Gas flow meters have been developed to determine how much gas is flowing through the pipeline. An orifice meter is one established meter to measure the amount of gas flow. More recently, another type of meter to measure gas flow was developed. This more recently developed meter is called an ultrasonic flow meter.
FIG. 1A shows one type of ultrasonic meter suitable for measuring gas flow. Spoolpiece 100, suitable for placement between sections of a gas pipeline, has a predetermined size and thus defines a measurement section. Alternately, a meter may be designed to attach to a pipeline section by, for example, hot tapping. As used herein, the term “pipeline” when used in reference to an ultrasonic meter may be referring also to the spoolpiece or other appropriate housing across which ultrasonic signals are being sent. A pair of transducers 120 and 130, and their respective housings 125 and 135, are located along the length of spoolpiece 100. A path 110, sometimes referred to as a “chord” exists between transducers 120 and 130 at an angle θ to a centerline 105. The position of transducers 120 and 130 may be defined by this angle, or may be defined by a first length L measured between transducers 120 and 130, a second length X corresponding to the axial distance between points 140 and 145, and a third length D corresponding to the pipe diameter. Distances D, X and L are precisely determined during meter fabrication. Points 140 and 145 define the locations where acoustic signals generated by transducers 120 and 130 enter and leave gas flowing through the spoolpiece 100 (i.e. the entrance to the spoolpiece bore). In most instances, meter transducers such as 120 and 130 are placed a certain distance from points 140 and 145, respectively. A fluid, typically natural gas, flows in a direction 150 with a velocity profile 152. Velocity vectors 153-158 indicate that the gas velocity through spool piece 100 increases as centerline 105 of spoolpiece 100 is approached.
Transducers 120 and 130 are ultrasonic transceivers, meaning that they both generate and receive ultrasonic signals. “Ultrasonic” in this context refers to frequencies above about 20 kilohertz as required by the application. Typically, these signals are generated and received by a piezoelectric element in each transducer. To generate an ultrasonic signal, the piezoelectric element is stimulated electrically, and it responds by vibrating. This vibration of the piezoelectric element generates an ultrasonic signal that travels across the spoolpiece to a corresponding transducer of the transducer pair. Similarly, upon being struck by an ultrasonic signal, the receiving piezoelectric element vibrates and generates an electrical signal that is amplified, digitized, and analyzed by electronics associated with the meter.
Initially, D (“downstream”) transducer 120 generates an ultrasonic signal that is then received by U (“upstream”) transducer 130. Some time later, U transducer 130 generates a return ultrasonic signal that is subsequently received by D transducer 120. Thus, U and D transducers 130 and 120 play “pitch and catch” with ultrasonic signals 115 along chordal path 110. During operation, this sequence may occur thousands of times per minute.
The transit time of the ultrasonic wave 115 between transducers U 130 and D 120 depends in part upon whether the ultrasonic signal 115 is traveling upstream or downstream with respect to the flowing gas. The transit time for an ultrasonic signal traveling downstream (i.e. in the same direction as the flow) is less than its transit time when traveling upstream (i.e. against the flow). In particular, the transit time t1, of an ultrasonic signal traveling against the fluid flow and the transit time t2 of an ultrasonic signal travelling with the fluid flow is                generally accepted as being defined as:                               t          1                =                  L                      c            -                          V              ⁢                              x                L                                                                        (        1        )                                          t          2                =                  L                      c            +                          V              ⁢                              x                L                                                                        (        2        )            where,        c=speed of sound in the fluid flow;        V=average velocity of the fluid flow over the chordal path in the axial direction;        L=acoustic path length;        x=axial component of L within the meter bore;        t1=transmit time of the ultrasonic signal against the fluid flow; and        t2=transit time of the ultrasonic signal with the fluid flow.        
The upstream and downstream transit times are typically calculated separately as an average of a batch of measurements, such as 20. These upstream and downstream transit time averages may then be used to calculate the average velocity along the signal path by the equation:                     V        =                                            L              2                                      2              ⁢              x                                ⁢                                                    t                1                            -                              t                2                                                                    t                1                            ⁢                              t                2                                                                        (        3        )            with the variables being defined as above.
The upstream and downstream travel times may also be used to calculate the speed of sound in the fluid flow according to the equation:                     c        =                              L            2                    ⁢                                                    t                1                            +                              t                2                                                                    t                1                            ⁢                              t                2                                                                        (        4        )            
To a close approximation, equation (3) may be restated as:                     V        =                                            c              2                        ⁢            Δ            ⁢                                                   ⁢            t                                2            ⁢            x                                              (        5        )            where, Δt=t1−t2  (6)So to a close approximation at low velocities, the velocity v is directly proportional to Δt.
Given the cross-section measurements of the meter carrying the gas, the average velocity over the area of the meter bore may be used to find the volume of gas flowing through the meter or pipeline 100.
In addition, ultrasonic gas flow meters can have one or more paths. Single-path meters typically include a pair of transducers that projects ultrasonic waves over a single path across the axis (i.e. center) of spoolpiece 100. In addition to the advantages provided by single-path ultrasonic meters, ultrasonic meters having more than one path have other advantages. These advantages make multi-path ultrasonic meters desirable for custody transfer applications where accuracy and reliability are crucial.
Referring now to FIG. 1B, a multi-path ultrasonic meter is shown. Spoolpiece 100 includes four chordal paths A, B, C, and D at varying levels through the gas flow. Each chordal path A-D corresponds to two transceivers behaving alternately as a transmitter and receiver. Also shown is an electronics module 160, which acquires and processes the data from the four chordal paths A-D. This arrangement is described in U.S. Pat. No. 4,646,575, the teachings of which are hereby incorporated by reference. Hidden from view in FIG. 1B are the four pairs of transducers that correspond to chordal paths A-D.
The precise arrangement of the four pairs of transducers may be more easily understood by reference to FIG. 1C. Four pairs of transducer ports are mounted on spool piece 100. Each of these pairs of transducer ports corresponds to a single chordal path of FIG. 1B. A first pair of transducer ports 125 and 135 includes transducers 120 and 130 recessed slightly from the spool piece 100. The transducers are mounted at a non-perpendicular angle θ to centerline 105 of spool piece 100. Another pair of transducer ports 165 and 175 including associated transducers is mounted so that its chordal path loosely forms an “X” with respect to the chordal path of transducer ports 125 and 135. Similarly, transducer ports 185 and 195 are placed parallel to transducer ports 165 and 175 but at a different “level” (i.e. a different radial position in the pipe or meter spoolpiece). Not explicitly shown in FIG. 1C is a fourth pair of transducers and transducer ports. Taking FIGS. 1B and 1C together, the pairs of transducers are arranged such that the upper two pairs of transducers corresponding to chords A and B form an X and the lower two pairs of transducers corresponding to chords C and D also form an X.
Referring now to FIG. 1B, the flow velocity of the gas may be determined at each chord A-D to obtain chordal flow velocities. To obtain an average flow velocity over the entire pipe, the chordal flow velocities are multiplied by a set of predetermined constants. Such constants are well known and were determined theoretically.
Thus, transit time ultrasonic flow meters measure the times it takes ultrasonic signals to travel in upstream and downstream directions between two transducers. This information, along with elements of the geometry of the meter, allows the calculation of both the average fluid velocity and the speed of sound of the fluid for that path. In multi-path meters the results of each path are combined to give an average velocity and an average speed of sound for the fluid in the meter. The average velocity is multiplied by the cross sectional area of the meter to calculate the actual volume flow rate.
Because the measurement of gas flow velocity and speed of sound depend on measured transit time, t, it is important to measure transit time accurately. More specifically, a characteristic of ultrasonic flowmeters is that the timing precision required is generally much smaller than a period of the ultrasonic signal. For example, gas ultrasonic meters have a timing precision on the order of 0.010 μs but the ultrasonic signal has a frequency of 100,000 to 200,000 Hz, which corresponds to a period of from 10.000 to 5.000 μs. Various methods exist for measuring transit times of ultrasonic signals.
One method and apparatus for measuring the time of flight of a signal is disclosed in U.S. Pat. No. 5,983,730, issued Nov. 16, 1999, entitled “Method and Apparatus for Measuring the Time of Flight of A Signal”, which is hereby incorporated by reference for all purposes.
A difficulty that arises in measuring a time of flight exactly is defining when an ultrasonic waveform is received. For example, a waveform corresponding to a received ultrasonic signal may look like that shown in FIG. 2. The precise instant this waveform is deemed to have arrived is not altogether clear. One method to define the arrival instant is to define it as a particular zero crossing but to get a good transit time one needs to find a consistent, reliable zero crossing to use. One suitable zero crossing follows a predefined voltage threshold value for the waveform. However, signal degradation due to pressure fluctuations or the presence of noise may cause the correct zero crossing to be misidentified, as shown in FIG. 3 (not to scale). Other methods for identifying arrival time may also be used, but each is also subject to measurement error by misidentification of the proper arrival time. An approach to determine whether a peak selection error has occurred is disclosed in U.S. Ser. No. 10/038,947, filed Jan. 3, 2002 and entitled “Peak Switch Detector for Transit Time Ultrasonic Meters”, which is hereby incorporated by reference for all purposes.
Although the problem of misidentification of an arrival time for an ultrasonic signal has long been known, previous approaches to identifying the instant of arrival for an ultrasonic signal are inadequate. There remains a need for a user-friendly ultrasonic meter and method that uses the diagnostic ability of the meter to check for malfunction in transit time measurements and automatically correct for it. Ideally, if the meter is working correctly, the meter would advise of any external anomalies (like bad flow profile, pulsation, etc.) in the rest of the metering system. Such a meter would provide improved performance over previous ultrasonic meters for measuring fluid flow, would maintain good performance, would advise if maintenance was necessary, and would alert a user to problems in the metering system or a need for recalibration. Also ideally, such a method or meter would be compatible with existing meters and would be inexpensive to implement.